Composing your letter - what to do.
As we've already insisted, all you actually have to include in your letter of resignation is the fact that you're quitting and the date that it will take effect. But you may want to make a point to your employer or you may feel that some sort of explanation is required. Both are fine; but there is a right way and a wrong way to do either.
If you just want to explain why you're leaving, then make it as brief and unemotional as possible. A number of suggested resignation letters are posted at www.resignationletters.biz, in order to give you an idea of how to phrase your remarks. Try to make your statements correct but general.
There are any number of writers on this subject, and there seems to be a consensus among them that even the most innocent phrase may cause the wrong conclusion to be taken - to your detriment later on should future employers consult your present one for references or general information. (In many places this can't be legally done but it may well happen informally anyway). For instance: "I'm taking a post with more challenging responsibilities". So - you weren't satisfied with what you were doing here? Malcontent worker! Self-protection can be taken so far that it becomes paranoia, and if you work at it you can discover dark meanings in even the most innocent remark. It's doubtful that most management types are dedicated diggers-out of subtle meanings - surely they have enough to do without that! - so it should be sufficient that you give your reasons in common terms and get specific only on uncontrovertible facts. You can say you're quitting because you're moving to Seattle, but not that you're moving to Seattle because the pay is better there.
If you're out to make a point, again be careful with your phrasing. If nepotism or favoritism is driving you away, it's pointless to say so until management changes, but you might say something like "the opportunities for advancement seem to be limited". Generally, if you can express an opinion, or make your statement seem like an opinion, you're probably in the clear, as long as you're talking about purely objective business matters. Nobody could argue with "the current downsizing has convinced me that I must look to my own future". If you are one of the unfortunates who have to work overtime on very short notice, you can probably get away with "I find that I can't continue to comply with the company's overtime policies". But still, do be cautious, say exactly what you mean, and STOP. There are cases where too much is better than too little, but this isn't one of them.
These articles have emphasized the importance of minimizing the emotional turmoil at the company when you leave. Not to be overlooked as a part of this is the self-esteem of your boss. If a number of staff have resigned in a relatively short period of time, it reflects badly on him or her. Even a single, isolated resignation - yours - may imply more than you think it does. So your letter should speak to him or her as one adult to another: you feel it necessary to depart but you still respect your boss, as a person if not as a manager. It's not necessary to be explicit about this but the phrasing you use should give this impression. Saving his/her feelings like this might not convey any immediate benefit but at the very least will not make things worse.